How I can add a control to a hand rig for easy hand poses?
You may do it using a Copy Rotation bone constraints and drivers.


I'll show you how to do it using one finger and then you may implement it to the whole hand. Add the Copy Rotation constraint to last two bones, so the lower bone copies the middle bone rotation and then middle bone copies the top bone rotation.



Set the constraint's spaces to Local&lt;-->Local for each constrained bone. Now, when you rotate the upper bone the other ones follow to create a fist.


Add a circle (it may be a bone with a custom shape or other object). It'll be your controller. Select the top bone (the one without the constraint), go to the Transform panel and add a driver to the rotation value (in my case a Z axis rotation). Hover the cursor over the value, press the RMB-->Add Single Driver. Go to the Graph Editor and set its mode to Drivers. Press N to open the properties panel. In Drivers panel choose the Transform Channel and Average Value options, set the circle as a controller in the Object/Bone window. Change the space to Local and type to Scale. Use Generator modifier to adjust things. Press Update Dependencies button at the end. Now scaling the circle affects the rotation of the bones.